When I first moved here long-term (as I've been in and out of Hong Kong for over 10 years but have been living here continuously for about 6) there were a lot of "creature comforts" I wished I stocked up on (like you mentioned goldfish crackers). But, nowadays a lot of those can be located (sometimes requires a bit of searching) somewhere in Hong Kong (yes, we do have goldfish crackers). The big difference you'll find is price. We've got lots of luxury shopping here in Hong Kong and when it comes to baby and children's stuff, there's no exception. So, those BPA-free food holders will be available but you may have to pay 3-5 times more to buy them here in Hong Kong. Of course, this depends on a lot of things including brand-name and how rare the item is.
Nowadays, the things I wish I could easily get from the States include:
-Clothes. For myself--I don't have the proportions of a Chinese woman and I find that the standard places to buy "foreigner" clothes--especially dress clothes are extremely expensive for me...having said that I've found a few export clothes outlet stores that carry non-brandname nice dress/business clothes that I go to for my work clothes...it just takes a lot more time and effort in Hong Kong to "source" items. There is rarely one-stop shopping for anything in Hong Kong--also, not having a car and getting around everywhere by public transport makes shopping a big headache sometimes. Also, the styles here in HK are vastly different from the States so if you like a particular brand or fit of clothes, I say, bring extra.
-Shoes--again, I have large feet so forget trying to find womens' shoes that are 1) well-made 2) affordable and 3) attractive in Hong Kong. But, I have had some shoes made for me at a shoe shop here for about $50 USD. They aren't exactly well-made and don't last for years but they work. The thing I have problems finding are cute athletic shoes. Of course, I can wear men's shoes but that gets old after 6 years. I've been literally laughed out of Hong Kong shoe shops when I show them my shoe size and ask if they have anything for women. I've also been handed hideous men's loafers like, "Sorry, honey, this is all we could come up with." So, anytime I go to the States I STOCK UP on shoes. And I can wear out a pair of shoes every 6 months in Hong Kong because I just walk so much here compared with the States. Having said that, I know some women buy their shoes online and have them shipped. For me, because of how my feet are, it's not so simple so I need to try a shoe on first to know if it fits.
-Health items--especially supplements. I use a supplement called chorella. I looked for over a year for chorella tablets in Hong Kong with no success. Finally, when my sister came for a visit I had her bring some with her. So, this type of thing would be available at any high-quality health food store in the States or if not, online. However, I've heard that there is a website out of Thailand called iHerb that carries certain products like that (although, I don't think they have chorella).
In the end, I'd say that when it comes down to it, the biggest issues is cost. You can get most things here in HK but you're going to have to pay much more for them than you're used to in the States and it's likely you're going to have to run all over the place to find what you need. But, by all means, if you have a favorite food (such as a tea or coffee that you like) then bring a little extra. Or, if you want to see what Hong Kong has to offer as far as food items go to this website:
http://www1.parknshop.com/WebShop/GuestVisitpage.do After you login as a guest then do a browse through their items. This is one of the biggest grocery chains in Hong Kong (they deliver for free if you buy more than $65 USD worth of food--we have food delivered about twice a month). It will give you an idea of what is commonly available--although there are other chains such as Taste, 360 and CitySuper that will have hard-to-find items sometimes.
After living here this long I've come to realize that most of the things I want are kind of passing fancies and aren't as important as I thought they were. If you have a good food processor, blender, juicer, breadmaker--those types of things, although you can get them in Hong Kong, I recommend bringing your own.
Hope these things help.
